1. The Field of the Invention
The present invention relates to optical devices such as isolators, optical attenuators and optical filters for use in the fields of optical communication, optical measurement and other optical applications.
2. Related Art
It is known that semiconductor lasers used as light sources for various optical systems become unstable in their oscillation by return lights from the optical systems connected to the semiconductor lasers. To prevent this, optical isolators are widely used. With the rapid expansion of optical communication systems in recent years, demand for production of compact isolators at a low cost is more and more increasing.
The basic construction of the conventional optical isolator is depicted in FIG. 6 which includes a Faraday rotator 11, a pair of polarizers 10a and 10b arranged on opposite sides of the Faraday rotator 11, and a magnet 12 disposed around the Faraday rotator for magnetizing the Faraday rotator 11. A pair of collimator lenses 9a and 9b are disposed between the polarizers and optical fibers 8a and 8b, respectively. With this construction, the light incident in the forward direction (the direction of arrow C in the figure) passes through the optical isolator and the light incident in the reverse direction (the direction of arrow D in the figure) is cut off whereby the function as an isolator is materialized but this conventional structure requires many optical parts and the overall size was large.
On the other hand, another conventional optical isolator was proposed to avoid such complicated structure by adopting simplified optical parts such as optical isolators, optical attenuators or optical filters embedded in substrate plates.
These optical parts are called “embedded type” optical parts and are each produced by embedding an optical fiber in a substrate, fixing it with a resin, cutting a groove in the substrate transversely across the optical fiber with a dicing saw or other means, inserting an optical element into the groove and fixing it with an adhesive. This construction attains an advantage of eliminating the necessity of aligning the light beam axes each other in the manufacturing of the optical isolators and thus the production is facilitated On the other hand, there is a drawback in this type that diffraction of light occurs when the light exiting from the optical fiber passes through the optical element and the insertion loss becomes large due to the required thickness of the optical element. To cope with this problem, another conventional optical isolator was proposed to enlarge the core diameter of the optical fiber by locally heating the optical fiber in order to reduce the diffraction.
In order to materialize the optical isolator disclosed in the above-mentioned patent publication, a TEC fiber which is a core enlarged optical fiber produced by locally heating the fiber is embedded in position in a capillary ferrule as an optical fiber-supporting member. Thereafter, the opposite end surfaces of the capillary ferrule are subjected to PC finishing and a groove for embedding the optical isolator element composed of a Faraday rotator and a pair of optical polarizers bonded on opposite surfaces of the rotator is formed by cutting the ferrule through the enlarged core portion transversely of the optical fiber. It is necessary for the optical isolator inserted in the groove has a size larger than the mode field diameter but as the mode field diameter at the groove is about several tens μm, it is sufficient that the size of the element is at least 100 μm. The size is desirably as small as possible in order to reduce the cost of the elements by obtaining multiple optical elements from the same stock of materials.
When a small optical element is inserted in the groove, a large void space remains in the groove around the element and the space is filled with an adhesive and the optical element is thus covered by the adhesive. Accordingly, there was a problem that the curing shrinkage of the adhesive or external thermal shock the adhesive tends to be deteriorated, leading to peeling of the adhesive from where a large amount of the adhesive is present. Also, it was not easy to position such small element at a proper location and bond it in position.
Accordingly, the present invention aims at providing an optical isolator which is easy to handle and has a high reliability.
Since embedded type optical isolator which uses polarizer glass is of polarization-dependent type, it is necessary to adjust the direction of the polarization of the incident light when the optical isolator is used for a LD module. This adjustment requires tools and apparatus as well as a number of steps, leading to increase in the cost of assembling the LD module. Although there is a method of indicating the position of light incidence plane with an ink or laser marker, the precision of such marking and positioning becomes low due to the compact size of the parts and thus a precise alignment of the incident plane of polarization is difficult to achieve.
Further, with general optical isolator, the surface of the optical element is inclined with respect to the light beam axis as a measure to remove the return light reflected from the incident surface of the optical and to compensate for the inherent deviation of the position of the outgoing light from the incident light. However, in the embedded type optical isolator, the inclined rectangular groove in which the optical element is inserted increases the light connection loss.
A larger angle of the inclination of the groove in which the optical element is inserted is required for a less return light but this measure leads to a greater connection loss. Accordingly, it is necessary to calculate the required reflection reduction and the allowable connection loss in design and to machine a groove with a best precision satisfying these factors.
However, when the groove machining is carried out using a diamond blade, one encounters a problem that the blade tends to walk or be deviated when the blade contacts the work (ferrule) as well as when the blade comes out of the work, with the result that the groove is dulled to an extent that a truncated trapezoid (divergent upwardly) in cross section is formed.
Further, the side wall surface of the groove on which the light is incident has a larger surface roughness because it has been cut by the diamond blade, and thus fine voids are likely to be trapped between the cut surface and the optical element to be embedded when the optical element is inserted in the groove and bonded with an adhesive, thereby causing a loss and reflection. Accordingly, it is necessary to mirror-finish the side walls of the groove.
As a method for finishing the cut side wall surfaces, so-called “float slicing” method is available, which comprises steps of simultaneously cutting and grinding the surfaces while supplying a lapping liquid (containing cerium oxide, colloidal silica, etc) to the cutting wheel and the work. However, this process enlarges the width of the grooves at the upper portion of the side walls of the groove to an larger extent larger as compared with the diamond blade cutting method.
Thus, the grooves obtained by this method is deviated from the designed cross section and thus there is a possibility that the reflection reduction and the connection loss are aggravated.
Explaining this problem in reference to FIGS. 7(a) and (b), which illustrate one method, even if a groove 23 in which an optical element is to be embedded is machined in a ferrule 20 supporting an optical fiber 21 therein with a desired angle of inclination as seen in the plan view of (a), angles θ1 and θ2 (usually θ1=θ2) of the side walls 25, 27 of the groove 23 formed by the machining fluctuation are introduced as seen from the lateral view of (b), which lead to a larger connection loss due to the larger angles than originally designed.
When a ferrule capillary having a function of optical fiber connector is used as a support for a terminal enlarged core (TEC) optical fiber and an optical element, the grooved portion of the ferrule capillary is physically weak and, especially when it is expected that the ferrule is repeatedly mounted and dismounted, it is difficult to use it as an optical connector. Thus, another object of the present invention is to improve the strength of the grooved portion of the ferrule capillary and providing an optical part having a structure capable of withstanding the repeated mounting and dismounting.